The recent development of copper-oxide perovskite superconductive materials having normal/superconductive critical transition temperatures ("T.sub.c ") significantly higher than previously known superconductive materials has spawned widespread interest in developing electronic devices which incorporate such perovskite superconductive materials and make use of superconductivity phenomena. The fabrication of semiconductor electronic devices which are generally planar in structure and comprise patterned layers of thin films of different materials is a well developed technology. It has been proposed to develop high-T.sub.c superconductive electronic devices which are generally planar in structure and comprise patterned layers of thin films of superconductive materials and insulative materials.
An article by Kingston et al. published in IEEE Transactions, on Magnetics, volume 27, pages 974-977 (March 1991 ) ("the Kingston et al. publication") disclosed a superconducting flux transformer fabricated as a thin-film YBa.sub.2 Cu.sub.3 O.sub.7-x --SrTiO.sub.3 --YBa.sub.2 Cu.sub.3 O.sub.7-x multilayer structure. The flux transformer included what were termed "crossovers" and "window contacts." A "crossover" was described as superconducting thin film wires of YBa.sub.2 Cu.sub.3 O.sub.7-x separated by an electrically insulating layer of SrTiO.sub.3. A "window contact" was described as a superconducting connection between two thin-film layers of superconducting YBa.sub.2 Cu.sub.3 O.sub.7-x made via a window opening patterned in an intervening layer of SrTiO.sub.3. Each of the three layers of the flux transformer of the Kingston et al. publication was deposited by laser deposition and was patterned photolithographically by etching through a correspondingly patterned layer of developed photoresist. Films of YBa.sub.2 Cu.sub.3 O.sub.7-x were patterned by a standard photolithographic process using either a nitric acid solution or an argon ion mill to etch the film. According to the Kingston et al. publication, exposure of the surface of YBa.sub.2 Cu.sub.3 O.sub.7-x to the photoresist of the photolithographic process generally left a layer of contamination that often prevented epitaxial growth of subsequent layers. In addition, photolithography followed by an etch tended to produce sharp edges in the patterned YBa.sub.2 Cu.sub.3 O.sub.7-x film which tended to be difficult to insulate. It was indicated that both the problem of sharp edges and the problem of surface contamination by photoresist could be ameliorated by etching the patterned surface with a solution of bromine in methanol.
According to the Kingston et al. publication, to make a window contact through a layer of SrTiO.sub.3 film which was sufficiently thick to be compatible with the crossovers of the flux transformer, the window opening had to have beveled walls. Such beveled walls in a window opening through an SrTiO.sub.3 layer was formed by forming a window opening in a layer of photoresist on the SrTiO.sub.3 layer which had sloping walls by a process which involved exposing the photoresist to an out-of-focus image of a window. The layer of SrTiO.sub.3 was then etched through the window opening in the photoresist using an ion mill to cut through the SrTiO.sub.3 layer to expose a lower layer of YBa.sub.2 Cu.sub.3 O.sub.7-x. According to the Kingston et al. publication, too little etching with the ion mill left an insulating layer, while too much could could degrade the quality of the contact. It was determined when to terminate the ion milling by examination of the window under a microscope. To reduce damage to the exposed YBa.sub.2 Cu.sub.3 O.sub.7-x, the ion milling was terminated by milling at a lower voltage for a few minutes. The bromine etch was not used after cutting the window through the SrTiO.sub.3 layer because the bromine solution tended to remove YBa.sub.2 Cu.sub.3 O.sub.7-x under the SrTiO.sub.3 layer.
Various etchants for Y--Ba--Cu--O films are noted in an article by Shokoohi et al. published in Applied Physics Letters, volume, 55, pages 2661-2663 (18 Dec. 1989). The use of solutions of phosphoric acid, nitric acid, and hydrochloric acid in water as acid etches in wet chemical etching was noted. It was disclosed that a saturated solution of ethylenediaminetetraacetic acid ("EDTA") in water was a suitable etch for use in standard photoresist lithography for microfabrication of superconducting devices. According to the publication, the EDTA solution readily removed Y--Ba--Cu--O, leaving substrate material intact.
An article by Jia and Anderson published in the Journal of Materials Research, volume 4, pages 1320-1325 (November/December 1989 ) ("the Jia and Anderson publication") disclosed that the resistance of Y--Ba--Cu--O superconductors to degradation by water was improved by a chemical treatment with hydrofluoric acid (HF) solution. Immersion of Y--Ba--Cu--O superconductor in HF was reported to result in a passivation of the surface of the superconductor. According to the Jia and Anderson publication, a thin layer of amorphous fluoride formed on the surface of the Y--Ba--Cu--O during HF treatment which limited later reaction between Y--Ba--Cu--O and water. The layer of amorphous fluoride also limited further reaction between. Y--Ba--Cu--O and HF. According to the Jia and Anderson publication, no noticeable etching occurred after 20 h of immersion of Y--Ba--Cu--O in both bulk and thin-film form in 49 percent HF. In the case of thin films of Y--Ba--Cu--O, the Jia and Anderson publication disclosed that the formation of an amorphous fluoride layer on the surface of the film made electrical measurements difficult. To avoid the effects of the amorphous layer formed during HF treatment under electrical contacts to the Y--Ba--Cu--O thin film, the contacts were evaporated onto the film before the HF treatment and the treatment was carried out with wet HF gas. According to the Jia and Anderson publication, when contact leads for measuring the resistance of a sample of Y--Ba--Cu--O treated with 49 percent HF were connected directly to a surface of the sample, the measured resistance versus temperature curve became unstable due to the formation of fluoride compounds on the sample surface. Scraping the surface layer before introducing the contacts avoided the instability.
It is generally desirable in the photolithographic patterning of multilevel thin-film superconductor/insulator structures for an etchant to exhibit a reasonably low etch rate with respect to the material it is desired to pattern and a high selectivity against etching other materials in the structure. A reasonably low etch rate is in part a matter of convenience but also tends to reduce undercutting under photoresist stencils in cases in which patterns are overetched to ensure that thickness variations are accommodated. Dry etching tends to be better than wet etching for avoiding stencil undercutting, However, the dry etching process of ion milling that has been used for patterning high T.sub.c films has poor selectivity. The poor selectivity of ion milling generally requires that the ion milling system be repeatedly vented for microscopic examination of samples for end point detection. Even so, sample thickness and/or milling rate nonuniformities sometimes preclude achieving satisfactory results over all regions of a wafer. The need for selectivity is particularly great for etching of multilayer thin-film high T.sub.c superconductor/insulator structures because one of the currently preferred methods for thin film deposition, laser ablation, tends to have poor thickness control and uniformity. In addition, the need for selectivity increases as substrate size increases and device geometries get smaller. As is the case for patterning films of high-T.sub.c superconductive materials, there is a need for high selectivity in patterning thin-film insulative materials which are compatible with the superconductive materials. Heretofore, no entirely satisfactory method for etching such insulative materials in the presence of perovskite superconductive materials has been available.